The present invention relates to ion guns and mass spectrometers. Mass spectrometers offer many benefits for the analysis of unknown gases, either for composition or for trace contaminants, however they have previously been regarded as complex and expensive. The subject of this patent application is a new design ion gun and of mass spectrometer that is relatively simple and compact which should extend the usage of mass spectrometers into new areas.
Mass spectrometers start by vaporising a sample, if not already in the gas phase, and ionising atoms or molecules in the resulting gas to form ions. These atomic or molecular ions are then manipulated by means of electric or magnetic fields, within a vacuum to prevent collisions with ambient gas molecules, in such a way that ions of different masses may be distinguished and their abundance measured. As each element has a different and unique mass the resulting "mass spectrum" may often be relatively easily interpreted in terms of concentrations of different elements. When molecular ions are involved the interpretation may be more complex because a single compound may give rise to several mass peaks due to fragmentation, however there exist databases of mass spectra for most compounds of interest. In particular there is a large body of mass spectral data [(NBS/EPA (USA) MS library (44,000 electron impact mass spectra)] associated with ionisation by means of electron impact.
By comparison with other analytical techniques, for example infra red spectroscopy, mass spectrometry has great advantages because of its applicability to a wide range of compounds together with its high specificity. Unlike most other techniques mass spectrometry allows different isotopes of the same element to be distinguished. It is also particularly well suited to use with a primary separation technique such as gas chromatography, as proposed by G. Matz et al, Chemosphere 15 (1986) p2031.
Mass spectrometers for gas analysis generally consist of a source of ions, a spectrometer where separation according to the mass-to-charge ratio takes place and an ion detector. All mass spectrometers have an evacuated chamber so that the mean free path of the ions of interest is much longer than their intended path within the spectrometer. There are various schemes for separating ions according to their mass-to-charge ratio and because the charge is generally known (e.g. the removal of a single electron) this equates to separation by mass. Most spectrometers effectively act as mass filters, arranging that only ions at, or near to, a certain mass complete the journey from ion source to detector. Examples of this technique are the magnetic or electrostatic sector instruments and Wein filter spectrometers which disperse the ions in space and either have a position sensitive detector or, more usually, a mass selecting aperture or slit. Quadruple spectrometers also work as a narrow bandpass filter, being arranged so that only ions of certain mass to charge ratio have stable trajectories and hence reach the detector. These filter type mass spectrometers can be used to create a mass spectrum by ramping the electric or magnetic fields in such a way that the mass detected is scanned through the range of masses of interest. When a signal from the detector has been collected throughout the range a mass spectrum may be plotted. Clearly when using this method only a small fraction of the ions created in the source actually reach the detector. Other types of mass spectrometer can in principle detect all the ions created in the source. Two examples are the ion trap and the time-of-flight mass spectrometer.
A number of factors affect the suitability of a particular spectrometer for a particular application: the constraints that it places on the source, such as range of ion energies accepted and the permissible physical source size; the ability to resolve small differences in mass; the transmission efficiency from source to detector; the range of masses covered and the complexity, and hence cost, of construction. Where a relatively small and inexpensive mass spectrometer has been required for gas analysis, by far the most common choice has been the quadruple mass spectrometer (see P. H. Dawson and N. R. Whetton, Advances in Electronics and Electron Physics, Chap III p60). Whilst it is possible to make these small and no magnetic fields or fine apertures are required, the quadruple does suffer a number of disadvantages: radio frequency power supplies are required, the mass range is usually rather limited, the mass resolving power is relatively low, the energy acceptance is only a few tens of volts, the source size must be fairly small compared with the spectrometer size, the transmission at any given mass is low, and it needs to be scanned to produce a spectrum. For these reasons other arrangements are increasingly being considered, in particular time-of-flight spectrometers.
In a time-of-flight mass spectrometer, as the name implies, the mass of an ion is deduced from the time taken for it to make the journey from source to detector. The transmission is usually not mass dependent over the range of interest and there is therefore no need for scanning. In addition the transmission efficiency may be quite high over a large range of source energy, for a physically large source and with good mass resolving power. The source needs to be pulsed in order to give a well defined start point for the ions, however apart from this, the remaining voltages may be static and hence require minimal power consumption. The arrangement of electrodes required is relatively simple and no magnetic fields are required, thus avoiding all the problems of weight, memory effect and non-linearity associated with magnetic materials. In principle the mass range is limited only by the length of time that the experiment is allowed to proceed after each pulse from the source. A recent readable review of time of flight technology is given by Cotter in Analytical Chemistry, 64 (1992) p1027.
Although time-of-flight spectrometers have been available commercially for some time, the MA-1 from the Scientific Instruments and Vacuum Division, The Bendix Corp. USA, for example, they are not widely used outside the analytical laboratory. This is because until relatively recently the electronics required for the timing measurement has been expensive and inconvenient to use. However the desire for very fast digital communications has now pushed electronics technology to the speeds required for this application.
When designing an electron impact ionisation source, the aims are: to have a high ionisation efficiency of the gas that is allowed in, to have efficient pumping of the source to remove any remaining neutral gas and to be matched to the spectrometer so that the ions produced are detected whilst maintaining the desired mass resolution. If the source is to be used for residual gas analysis then the source volume should be reasonably large so that a good number of gas atoms are available to be ionised. In practice these various requirements conflict. In particular it is difficult to have a large enough source volume to include many neutral species whilst at the same time getting: (a) an electron source close enough to give good ionisation, (b) ion extraction optics that are close enough to extract a beam of ions with dimensions that allow efficient transmission through the spectrometer at good mass resolution, which implies an ion beam narrow in at least one dimension and possibly two, unless the detector is to be rather large (c) a gas inlet, if there is one, close to the source region so that most of the neutral gas atoms/molecules emerging from the inlet pass through the ionisation region, and (d) the pumping used to remove excess gas close to the source region, preferably opposite the gas inlet, so that the gas that does not get ionised is pumped away immediately rather than finding its way into the rest of the spectrometer.